He just went to buy some cigs...
Why is "vandaag komt vader niet" incorrect?
And FYI because of the word order you chose, it has more emphasis on vandaag, similar to: Today, dad is not coming.
It would be correct, but it is more formal. The English translation would be "Father is not coming today."
Is it possible to say ‘Pa komt vandaag niet‘? If not, is there some rule/explanation?
Why not use geen?
Geen negates nouns (you can think of it as the negative form of een), niet is essentially used in all other situations. See: https://www.duolingo.com/comment/3734833